The invention relates generally to shock absorbers for the insertion in a drill, tubing, or wireline string to isolate downhole explosive apparatus from other downhole tools. In particular, the invention relates to a shock absorber for isolating the jarring effect of perforating guns and high-energy gas stimulation systems from delicate instrumentation or other downhole equipment that may be prone to mechanical damage from perforating gun detonation.
During oil or gas well completion operations, it is necessary to perforate the casing to provide communication between the hydrocarbon bearing formations and the wellbore, so that the hydrocarbons may be produced to the surface. The casing perforation operation is carried out using explosive shaped charges, which blast holes in the casing and its surrounding cement sheath to access the hydrocarbon bearing formation.
It is becoming common to place instrumentation packages in close proximity to the perforating guns. These instruments measure downhole pressures to provide an indication of the influx rate, pressure, or temperature of the wellbore to give an indication of the success of the perforations and the production rate of the well. The firing of the perforating guns or stimulation devices produces large shock waves, which exert huge forces upon the instrumentation or other downhole equipment, often causing their failure. A shock absorber, when positioned between the perforating gun and the downhole equipment, considerably reduces the forces upon the equipment and expands their operating range and useful lifetime. Shock absorbers also permit the deployment of certain other tools that otherwise may not be operated in conjunction with perforating systems.
Some prior art shock absorbers rely on elastomeric elements, formed of rubber for example, to absorb shock waves. However, the effectiveness of an elastomeric element is limited due to the limited range of motion of the element and its inherent damping characteristics. A typical elastomeric element might be able to withstand 0.5xe2x80x3 of travel, which is generally ineffective for use with perforating systems. Other shock absorbers have used spring and damper arrangements including compressible oil. Compressible oil is very expensive and transmits large loads to the instrumentation due to the high loading required to begin compression of the fluid and springs. The need exists for a shock absorber that is more effective and less expensive than those already in use.
A shock absorber for damping shock energy generated by downhole perforating guns or stimulation devices has been invented and is disclosed herein. The invention provides an apparatus for conducting downhole measurements while perforating wherein the impulsive energy of the perforating guns is absorbed and, thereby, shielded from the downhole measurement tools.
In accordance with a broad aspect of the present invention, there is provided a shock absorber for absorbing the energy generated by a perforating gun, comprising a spring assembly including at least a first spring and a second spring, the first spring having a tension greater than the second spring; a damper assembly; and a housing retaining the spring assembly and the damper assembly and including ends adapted for connection into a casing perforation assembly.
The shock absorber according to the present invention includes ends formed for attaching into a casing perforation assembly, the assembly including a perforation gun and a downhole tool desired to be shielded from the force exerted by the perforation gun during detonation such as, for example, a gauge recorder. Preferably the ends of the shock absorber are formed for threaded engagement into the perforation assembly. The shock absorber housing preferably includes two telescopically disposed parts between which the spring assembly and the damper assembly act.
The spring assembly is the primary mechanism for absorbing and dissipating the shock loading from the perforating guns. The shock absorber is designed to be double acting, so that the shock impulse can be applied from either direction and be absorbed and dissipated, without the need for a set of springs to handle impulse from each direction. The shock absorber includes a plurality of springs preferably of various stiffnesses, so as to give multiple discrete spring rates. In one embodiment, a plurality of springs are used, each of which is progressively stiffer and serves to gently absorb the shock wave. In a preferred embodiment some of the springs can be removed or further springs can be added to change the preload of the spring assembly. Changing the preload of the spring assembly provides that the shock absorber can be adjusted with consideration as to the downhole tools that are to be hung from the shock absorber. In this embodiment, the preload of the shock absorber can be adjusted to be in the neutral position when the downhole tools such as, for example, a gauge recorder, are hung from the shock absorber. The selection of preload allows maximum stroke and energy absorption from the shock absorber in service. In the preferred embodiment, the spring rates are fairly low so that the shock absorber can absorb most of the shock while the tools hung below the absorber remain relatively stationary during the detonation of the guns.
The damper assembly has a plurality of oil filled chambers and a valve mechanism to regulate the flow of oil from a first chamber to a second chamber. The oil is preferably substantially non-compressible to reduce the cost of the shock absorber over a shock absorber requiring compressible oil. Preferably, the first chamber includes a piston that is moveable to reduce the volume of the first chamber in response to a loading, such as that applied by the detonation of a perforating gun. When the piston reduces the volume, oil will be forced from the first chamber into the second chamber. In one embodiment, the chamber is selected such that its volume will only be reduced by a load over a selected level. In one embodiment, the damper valve mechanism includes a metering mechanism to restrict the flow of oil from one chamber to another in response to the loading. The damper assembly can be hollow, having a bore therethrough for permitting passage of a member such as, for example, a conductor (i.e. wireline) or full tubing bore therethrough and, thereby, through the centre of the shock absorber.
It is an object of the present invention to provide a mechanism for protecting the very delicate instrumentation, such as pressure recording gauges, when the perforating guns are detonated. It is a further object of the invention to provide a shock absorber that is capable of very rapid displacement and subsequent shock absorption and dissipation.